Storm Motion

When a hurricane is on the weather map, everyone wants to know where it will go. Only the people whom the storm will hit have an overwhelming concern with its winds, rainfall or storm surge. Everyone else wants ressuurance that it will miss. Forecasts are always uncertain. For individuals and enterprises there is invariably a tradeoff between the cost and effort of preparations and the probability of casualties. Hurricane conditions typically affect a swath of about 100 nautical miles wide; the error in a 24-hour forecast also is now somewhat less than 100 nautical miles. Thus, a prudent forecaster expects to raise warnings on 300 nautical miles of coastline: 100 nautical miles that actually feel the hurricane and 100 nautical miles on either side to allow for forecast error.

Warnings are expensive. The economic cost of raising a warning, in terms of lost productivity, safeguarding homes and industries, evacuation of aircraft and vessels, and canceled beachfront vacations averages $0.5 to 1.0 M per mile. The cost depends upon the amount of coastal development. If a major city, such as Miami or New Orleans, lies in the warning area, cost escalates dramatically. Sometimes, as happened in Hurricane Emily of 1993, models and observations combine to give forecasters particularly clear insight into the meteorological situation, so that they can exclude large sections of coastline, perhaps hundreds of miles long, from the warning area and save the economy as much as $100M. When, on the other hand, as happened in Hurricane Floyd of 1999, the storm track remains offshore running parallel with the coast, warnings extend far beyond the area affected, and overwarning costs are immense.

As a problem in fluid mechanics, the dominant factor in hurricane motion is the surrounding wind. In effect, the storm is carried downstream by the average environmental wind. It also "propagates" westward and poleward because it is on a rotating, spherical Earth. Local vertical is more nearly parallel with the planet's axis of rotation in high latitudes. "Beta effect" propagation arises from the poleward increase of the planet's rotation around local vertical. Air that moves around the the vortex center with the storm circulation tends to keep a constant value of the sum of the planetary and relative vorticity. Vorticity in the context is the tendency of the air to swirl around a vertical axis. It is the stuff of vorticies.

An anticyclonic asymmetry thus forms on the northeast side of the vortex and a cyclonic asymmetry on the southwest side. This happens because where the planetary vorticity is larger the relative vorticty must be smaller, and conversely. These "beta" gyres induce a northwestward flow across the vortex center that pushes the storm northwestward. Hurricane motion is 80 to 90% due to large scale winds and 10 to 20% due to propagation. Nonetheless, at a typical speed of 1 m s^-1, the motion due to propagation is comparable with typical forecast errors.

Over the last 30 years, the decrease in forecast error has averaged about 1% per year , largely because of improved computer models. In fact, the models are now better than the knowledge of the "initial condition," the state of the atmosphere at the start of a prediction calculation. Research at AOML has demonstrated that dropsonde observations from aircraft flying around hurricanes can lead to another 15% reduction in the error. If the more accurate forecasts translate into reduced warning areas, savings on the order of $10 to 20M in overwarning costs per hurricane landfall are possible. This dramatic result recognizes that hurricane motion is controlled by the state of the surrounding atmosphere, so that forecasts based upon accurate and timely measurements of that state are themselves more accurate.

In these "synoptic surveillance" experiments, the atmospheric state is measured by dropwindsondes deployed by the NOAA research aircraft. Sondes are released from an altitude of 5 to 7 kilometers. They measure pressure, wind, temperature, and humidity as they fall to the sea on parachutes. Release patterns encircle the storm and reach more than 1,000 kilometers from the center. Aboard the aircraft, meteorologists process the data and transmit it to the National Centers for Environmental Prediction, where it is analyzed and used to initialize numerical models. The cost of the flight operations is about $130,000--about one percent of the potential savings from reduced overwarning.

More widespread and efficient observations of this kind are the primary reason that NOAA commissioned its new Gulfstream IV jet that has flown synoptic surveillance missions operationally since 1997. The jet procurement included the new GPS dropsonde instruments themselves and new faster in-flight processing and data-visualization software. These improvements are important contributors to the accelerated decrease in forecast errors in the late 1990s.